Carbon-free shell molds



April 26, 1960 R. H. COOPER CARBON-FREE SHELL MOLDS Filed Sept. 26, 1956 mmvroa A'ano/ah. Cooper HTTORNEYS Unite States PatentfOl CARBON-FREE SHELL MOLDS Ronald H. Cooper, Clare, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application September 26, 1956, Serial No. 612,286

19 Claims. (Cl.221 93) This invention relates to shell molds that are substantially free from carbon and adapted, for such reason, to be utilized in a metal casting process with low carbon alloys. It has particular reference to improved coated sand compositions that are especially well suited for the fabrication of such shell molds and to substantially carbon-free shell molds that are prepared from such compositions. The invention also relates to a method for the fabrication of substantially carbon-free shell molds that utilizes the compositions.

Shell molding techniques represent a relatively recent development that is finding increasing favor for employment in foundry practice for metal casting operations. Most resin-bonded shell molds may be fabricated in a manner similar to that which is utilized in the so-called Croning process which has been described in F.I.A.T. Final Report No. 1168 (dated May 30, 1947), by the Field Information Agency, Technical, United States Department of Commerce. In shell molding, rigid thinwalled molds comprised of thermoset resin-bonded mixtures of sand or other fine grained refractory material are employed to hold the molten metal which is desired to be cast. The resin bound sand molds are ordinarily prepared by permitting a free-flowing granular mixture of sand and a suitable thermoplastic and thermosetting resin to come in contact with a metal pattern, usually under the influence of heat for a sufiicient period of time and, in some cases, under an applied pressure, to permit a layer of the mixture to adhere to the surface of the pattern and form a reproduction thereof while the resin is in a thermoplastic temperature range or while the composition is in an otherwise plastic and formable condition. The sand molds are then thermoset to form the desired strong and rigid structures having high gas permeability, good surface smoothness and exceptional dimensional stability.

Such mold forms, in which either ferrous or non-ferrous metals may be cast, frequently permit better castings to be obtained than by conventional methods due to their more porous structure and more precise surface finishes than ordinary sand molds. In addition, they oftentimes result in a more economical foundry practice than may be achieved with conventional sand molds since they require less sand than the usual green sand molds and can be fabricated with very small quantities of relatively inexpensive resin binders. Furthermore, they are lighter and less cumbersome and awkward to handle or manipu late than the traditional sand molds and eliminate much 2,933,789 lgc Patented Apr. 25, 1950 the usual shell molds are not well adapted for use with certain alloys and metal formulations of the high temperature and low carbon content varieties, especially when relatively thin-sectioned castings are desired. This is because such metal compositions, while in a molten condition at elevated temperatures, tend to assimilate carbon from the resin binder in the shell mold when it is attempted to fabricate them in the conventional shell molding processes.

As is well known, low carbon and high temperature alloy castings, especially in relatively thin sections, are extremely desirable for many purposes, particularly for many aircraft and engine parts. Great difliculty, however, has been experienced in being able to provide such castings by any means. By way of illustration, one technique for their manufacture is referred to as ceramic mold casting and is known as the Shaw process. This process requires the preparation. and use of ceramic molds which are extremely painstaking and tedious to prepare. Frequently the ceramic molds, after their formation, must be air dried from as long as three to four days and subsequently baked or fired for seven to ten days before they may be individually employed in a single casting operation. This technique, which has been described by Vogel in an article entitled Ceramic Mold Steel Castings, at page 100 of the January 1956 issue of Materials and Methods (vol. 43, No. 1), is not only hampered by its cumbrous and dispendious characteristics, but does not readily accommodate large scale production requirements.

It would be highly advantageous and consummately beneficial to provide a composition that would be particularly adapted to be fabricated into substantially carbonfree shell molds and the like structures that could be employed for the casting, in a more or less analogous manner to conventional shell molding procedures, of high temperature and low carbon alloys that require preservation from carbon contamination or assimilation during their molding into desired shapes.

These desiderations and many other salutary advantages and benefits may be achieved with a composition 1 in accordance with the present invention which is coma material.

of the hazard and unpleasantness due to dust and powprised of a preponderant proportion of sand or an equivalent refractory material in discrete particle form; a minor proportion of a fusible silica glass powder; and a binding quantity of a thermoplastic-thermosetting organic resinous The compositions may be provided either as dry, free-flowing mixtures or as wet-shapeable masses that may be formed into shell molds accordiing to conventional practices, such as by dump-box techniques- Thus, if they are provided as dry, free-flowing mixtures, they may be placed in contact with a heated pattern or otherwise heated to a temperature in their thermoplastic range while in contact with the pattern to be formed into a reproduction of the desired pattern by fusion of the resin binder before being thermoset by further application of heat to a rigid shell mold form. Alternatively,

and frequently with greater advantage, the compositions may be provided as plastic, flowable wet mixtures by employing a liquid resin binder. Such wet mixtures may be cold pressed, as by ramming, against the desired pattern or may be distributed on the pattern with the assistance of pneumatic pressure, much in the manner of conventional foundry core blowing practice, to initially form the desired shell mold formed. The initially formed shell molds may then be cured or thermoset to a rigid condition. After their initial formation by either propanying draying, are substantially free from carbon and a may be employed without difliculty in casting high temperature and low carbon metal alloys in the general manner of shell molding practice. Preferably, when such metal alloys are cast in shell molds according to the invention, casting is performed in an inert atmosphere a such as is a nitrogen or a suitable hydrocarbon atmosphere.

Advantageously, the compositions of the present invention may be comprised of sand with from 3 to 15 percent by weight of the thermoplastic-thermosetting organic resinous binder material, based on the weight of the composition and an amount in the same range of the fusible silica glass powder. More advantageously, the compositions, before they are formed into cured shell mold structures and subsequently fired to burn out the organic binder and fuse the silica glass powder, are comprised of an amount of the organic resin binder that is between about 6 and 12 percent by weight, based on the weight of the compositions, and a quantity of the fusible silica glass powder that is in the neighborhood of 10 percent by dry and free-floating compositions or it may be used in liquid form, either dissolved in a suitable solvent, molten, or as a liquid resin composition to wet coat the sand granules with a binding quantity of the resin. The liquid resin in a wet coated composition may, as has been indicated, be permitted or caused to set or harden to a thermoplastic-thermosetting coating to provide a dry freeflowing composition that may be hot formed on a desired shell mold pattern. Or, as has also been mentioned, the composition may be employed as a flowable and formable wet mixture with the applied resin in the liquid state. When the sand is being wet coated with the resin binder in liquid form, it is ordinarily desirable to intimately disperse the silica glass powder in the liquid resin prior to the sand coating operation.

Advantageously, the resin binder that is employed in the compositions of the present invention is a self-hardening liquid mixture of an aqueous phenolic liquid resin such as a phenol-formaldehyde liquid resin and a powdered magnesium oxide catalyst that is capable of dehydrating and auto-hardening the liquid resin at room temperatures to a dry thermoplastic-thermosetting mass. Such a resin binder for inert filler materials is described in the copending application of Ronald H. Cooper covering Improved Phenolic Resin Compositions having Serial 7 No. 612,283 that was concurrently filed on September weight, based on the weight of the composition. In many of such flux that it is between about 10 and 25 percent by weight of the fusible silica glass powder is ordinarily suitable.

Any ordinary sand or other refractory material in discrete particle form may be employed in the practice of the present invention. equivalent that is employed is a relatively fine grained material to facilitate the manufacture of smooth finish mold forms. Fine grained sand that has a fineness in accordance with the values proposed by the American Foundrymans Society (AFC) that is in the numerical range between about 75 and 180 may ordinarily be found suitable. Such sands, for example, as the types which are known as Berkeley Float sand, Janiata sand, Lake sand, Vassar sand, Wedron sand and the like may be beneficially employed. It is desirable that the sand be clean and substantially free from foreign metal oxides, clay, moisture and organic matter. In many cases, it may be more advantageous to employ a sand having an AFS fineness number from about 100 to 180. Very frequently, sands that have an AFS fineness number in the neighborhood of 100 may be satisfactory.

Conventional silica glass powders from boro-silicate types of glass, such as those comprised of about 96 percent by weight of silica glass, 2.5 percent by weight of boron trioxide and the balance inert materials may be employed in the practice of the invention: Beneficially, the silica glass powder that is used has a softening. point of about 1290 F. and is a finely divided material, such as one that has an average particle size of finer than about 100 mesh in the Us. sieve series. The best results may often be obtained when the amount of the silica glass powder that is employed does not exceed the quantity of the organic resinous binder that. is utilized in the composition.

Any thermoplastic-thermosetting organic resinous binder material that upon fusionin a thermoplastic. temperature range, is capable of binding sand granules into a coherent mass, may be employed. Ordinarily, phenol- Advantageously, the sand or its a 26, 1956, to issue as United States Letters Patent No. 2,869,194. Thus, the phenolic liquid resin that is employed may be a phenol-formaldehyde condensation product of the type that is oftentimes characterized as being a stage A resin that has been prepared by reacting aqueous mixtures of phenol and formaldehyde, in a known manner, under the influence of basic catalysts. Ordinarly, such liquid resins have a greater than 1:1 mole ratio of formaldehyde to phenol, respectively, in their compositions. It may frequently be desirable for a phenol-formaldehyde liquid resin to be employed that has a mole ratio of formaldehyde to phenol in the neighborhood of 1.45:1 with a solids content of at least 50 to 70 percent by weight, a viscosity from about 100 to 1,000 centipoises at 77 F. and a pH from about 5 to 9. The magnesium oxide catalyst that is incorporated in such a phenolic liquid resin to achieve its auto-hardening properties may be a finely divided powder that has initial setting characteristics, measured as a function of time according to the procedure set forth in A.S.T.M.

' Specification No. C254-50T, that is between about 0.1

and 6 hours.

The time that is required for a phenolic liquid resin of the indicated type to self-set due to the effect of the included powdered magnesium oxide powder depends to a great extent upon the activity or. initial setting time characteristics of the magnesium oxide powder and the proportion in which it is included with the phenolic liquid resin in the binder. This, of course, limits the time in i which a composition prepared with such a liquid resin catalyst and about 6 percent by weight of a liquid phenolformaldehyde resin containing about 70 percent by weight of solids in which the ratio of formaldehyde to phenol was about 1.45:1. 'The viscosity of the liquid resin at 77 F. was about -centipoises and the pH was about 5.

TABLE L-SEL'F-SETTING TIME OF'VARIOUS COATED BANDS AS AFFECTED BY INITIAL SETTING TIME OF The self-setting characteristics of a magnesium oxide catalyzed phenolic liquid resin coating on the sand are also influenced by the relative quantity of the catalyst that is present therein. Greater quantities of included catalyst generally shorten the initial self-setting time of the coating. A greater quantity of the catalyst also tends to alter the thermoplastic characteristics of the coated sand compositions after the resin coating has initially auto-hardened by raising the softening or melting point of the coating. This is illustrated in the following Table 2, wherein the proportions of various magnesium oxide catalysts were varied in several compositions with a phenolic liquid resin (similar to that utilized for deriving the data presented in Table 1) to determine the effect on the auto-hardening time at room temperature of the compositions.

TABLE 2..AUTO-HARDENING TIME OF COMPOSITIONS CONTAINING VARIED PROPORTIONS OF MAGNESIUM OXIDE CATALYST Composition G H .T K

Percent by Weight of Liquid Resin In Con position 94.40 Percent By Weight of MgO Cataly st 'In Corr position 5. 60 Percent By Weight 01' Catalyst In Composition Based On Resin 5. 94 Working Or Mixing Time Perrrissible With Corrposition Forn ulated With Hr. MgO Catalyst, Minutes; Same With Equivalent Composition Using 6 Hours MgO Catalyst Total Auto-Hardening- Tirre Of Composition Formulated With $4 Hour MgO Catalyst, rrinutes Same With Equivalent -Corrposition Using 6 Hours MgO Catalyst..- Melting r Softening Point In F.

of Auto-Hardened Composition Greater than 480.

Generally between about 10 and 25 percent by weight of magnesium oxide, based on the liquid resin content in the binder, is a suitable proportion to employ as a catalyst for aqueous phenolic liquid resins intended for application as. organic resin binders in the practice of the present invention. Advantageously, the powdered magnisum oxide that is employed has .an initial setting time between about 0.5 and3 hours and is so finely divided that its average particle size is finer than about 40 mesh in the U.S. sieve series. Preferably, the aqueous liquid phenolforrnaldehyde resin that is magnesium oxide catalyzed for utilization as a"thermoplastic-thermosetting resin binder in the practice of the invention has a solids content in the aqueous vehicle of about 50 to 70 percent by weight, a pH between about and 8 and a viscosity at 77 F. between about 300 and-500 centipoises.

As has been indicated, the utilization of magnesium oxide catalyzedphenolic liquid resins as binder materials in ;the practice of the present invention advantageously permits the shell mold fabricating compositions to be initially cold formed as a plastic and flowable wet mixture by a cold production technique which does not necessitate the'employment of heat or solvents in either the formulation of the coated sand compositions or the initial forma tion oftheshell mold reproduction of a desired patteiii. This benefit, as has been indicated, stems from the ability of magnesiumoxidecatalyzed phenolic resin compositions to auto-harden at room temperatures. In so doing, they may form an agglomerated, coherent mass, as when the sand composition is being pressed or otherwise distributed against thedesired pattern to cold form a shell mold, that may subsequently be thermoset 'to a rigid structure. 1 Or they may form a dry, free-flowing granular composition if they are efiiciently mulled or mixed during the auto-hardening of the applied liquid resin coating to prevent agglomeration of the coated sand.

Regardless'of the specific organic resin binder that is employed, it is essential to achieve a uniform and thorough dispersion and mixing of all the ingredients of the comp ositoin and tohomogeneously blend the resin binder and silica glass powder, either independently or in combination with the sand; When a magnesium oxide catalyzed phenolic liquid resin is being employed as a binder, it is particularly advantageous to prepare the composition by p'remixing the powdered magnesium oxide with the sand and, as mentioned, intimately dispersing the silica glass powder in the phenolic resin before thoroughly interblending and mixing the thus preliminarily prepared ingredients. If desired, however, the silica glass powder can also be premixed in the sand with the powdered magnesium oxide.

If a dry, free-flowing composition is employed, the shell mold may be formed readily by bringing the granular composition'in contact with a desired pattern and applying heat, as from a heated pattern, for a sufficient period of time to raise the temperature of the composition to the thermoplastic range of the resin binder. This, of course, may vary with the particular organic thermo plastic-thermosetting resin binder material that is employed. When a dry, auto-hardened magnesium oxide catalyzed phenolic liquid resin coating is present in the composition, the initial forming may usually be accomplished with pattern temperatures from about 425 to 500 F. with contact times of the coated sand composition on the pattern between 10 and 30 seconds. Wet mixtures may ordinarily be cold formed to shell mold reproductions of the desired pattern under ram pressures of about -100 pounds per square inch (gauge) or may be distributed by core blowing techniques over a pattern using pneumatic pressures in about the same range. With wet mixtures comprised of'magnnesium oxide catalyzed phenolic liquid resin hinders, the shell mold reproduction may ordinarily be cold formed in such a manner within an hour. In either case, as has been mentioned, somewhat conventional dump-box practice may be utilized for the initial shell mold formation if a core blowing technique is not employed for the wet mixture. After being formed, the shell mold reproductions are thermoset or cured to rigid structures under suitable time-temperature conditions for thermosetting the particular resin binder that is involved. Magnesium oxide catalyzed phenolic resin binders (which are usually thermoplastic in the temperature range from about to 250 F.) may ordinarily be thermoset at temperatures between about 250 and 600 F. or higher if shorter contact times are employed. Frequently, the curing of formed compositions containing such binders may be satisfactorily accomplished by exposing them to a temperature in the neighborhood of 475500 F. for periods of from at least 45 to 60 minutes.

The thermoset shell molds may then be burned free of the organic binder at silica glass fusing temperatures between about 1740 and 1900 F. for suitable periods of time to fuse the contained silica glass so that the rigid shell mold structure may be retained. This operation, as hasbeen indicated, transforms the shell mold structure to a suitable mold form for the casting of high temperature and low carbon alloys without danger of undesirable carbon pick up by molten metal during the casting opera tion. A firing temperature of about 1800+1850 F. for about /2 hour is generally satisfactory for fusing the silica glass powder in the thermoset shell mold form and burning out the organic resin binder utilized 'in. their preparation. H

The substantially carbon-free shell molds of the present invention and the methods for their formation and utilization provide, as is apparent, many advantages for the technique of casting high temperature and low carbon alloys. Their desirability for such purposes is especially prominent when they are contrasted and compared with ceramic mold forms. For example, carbon-free shell molds according to the invention may be prepared in mere fractions of the time that is required for the manufacture of a ceramic mold form as may bev represented by the contrast in time involved with a shell mold requiring only one or more hours for its fabrication compared with the ten or more days usually necessary for a ceramic mold. Besides this, shell molds are less brittle and subject to less breakage in handling than are the ceramic mold forms. In addition, after'they have been employed to produce a .casting, the carbonafree shell molds of the invention may be easily broken away and disintegrated from the molded metal form whereas ceramic molds are often diificult to remove from the finished casting. Furthermore, ceramic molds frequently tend to adhere to the poured metal casting whereas this problem is not encountered when castings are prepared with the carbon-free shell molds of the present invention.

The invention is further illustrated in and by the following example wherein, unless otherwise indicated, all parts and percentages are to be taken by weight.

Example A composition comprising about 81.2,parts of Vassar AFS 100 sand, 10 parts of silica glass powder, 8 parts of an aqueous liquid phenolic resin and 0.8 part of' /2 hour powdered magnesium oxide was prepared for fabrication into carbon-free shell molds in accordance with the present invention. The phenolic liquid resin was a phenol-formaldehyde condensation product that had a formaldehyde to phenol mole ratio of about 1.45:1, a solids content of about 50 percent, a pH of about 8 and a viscosity at' 77 F. of about 300 centipoises. The composition was formulated by thoroughly premixing the magnesiumv oxide powder with the sand and intimately dispersing the silica glass powder in the liquid resin before homogeneously blending the separate ingredients to form a plastic'flowable wet mixture. The wet mixture, before the applied resin had auto-hardened, was cold formed into a shell mold form by pressing it against a pattern under a pressure of about 80 pounds per square inch (gauge) while the resin binder was auto-hardening during a one hour period. After being cold set, the coherent shell mold forms were cured by being thermoset at a temperature of about 482 F. for periods of time of at least 45 to 60 minutes. They were then fired to fuse the silica glass powder to retain the rigid structure and burn out the resin binder at a temperature of about 1832" F. for a period of time of about minutes.

The resulting shell molds were substantially free from carbon. They were used successfully to cast low carbon stainless steel alloys in a nitrogen atmosphere with excellent results. The castings prepared with the shell moldings were precisely formed and had good surface finishes. In addition, their necessarily low carbon content remained unaffected during the casting procedure.

Similar results may also be obtained when the shell molds are initially formed by dropping the coated sand' ance of coated granules having an'average particle size,

finer than'about 40 mesh in the US. sieveseries. The shell molds may be so fabricated from the dry, freeflowing coated sand composition with pattern temperatures between about 425 and 500 F. and contact times of the composition onthe pattern between 12 and 20 seconds, The hot formed shell molds may then subsequently be cured and fired under about the same conditions as described above.

It is to be appreciated that, within the comprehension of the present invention, other desired refractory structures besides shell molds and the like may be. fabricated from the compositions and in accordance with the invention.

.Certain changes and modifications can be entered into readily in the practice of the present invention without substantially departing from its intended spirit and scope, Therefore, it is to be fully understood that the invention is not to be limited or otherwise restricted to or by the foregoing illustrative description and specification. Rather, it is to be interpreted and construed in the light of what is set forth and defined in the hereto appended claims.

What is claimed is: V

1. Composition for the fabrication of carbon-free shell molds and the like which consists essentially of a. preponderant proportion of sand; a minor proportion of between about 3 and 15 percent by weight, based'on the weight of the composition, of a fusible silica glass powder; and a binding quantity of a thermoplasticthermosetting organic resinous material, said resinous material consisting essentially of a mixture of (a) an aqueous liquid phenol-formaldehyde resin that has a greater than 1:1 mole ratio of formaldehyde to phenol, respectively, asolids content of at least about 50 percent by weight, a pH between 5 and 9 and a viscosity at 77 F. between about and about 1,000 centipoises and (12) between about 10 and 25 percent by weight, based on the weight of said resin, of an active powdered magnesium oxide having an initial setting time of less than about 6 hours.

2. The composition of claim 1 and including a minor quantity of soda ash flux for the silica glass powder.

3. Composition for the fabrication of carbon-free shell molds and the like which consists essentially of a preponderant proportion of sand; a minor proportion of between about 3 and 15 percent by weight, based on the weight of the composition, of a fusible silica glass powder; and between about 6 and 12 percent by weight, based on the weight of the composition, of a thermoplasticther-mosetting organic resinous binder material, said resinous material consisting essentially of a mixture of (a) an aqueous liquid phenol-formaldehyde resin that has a greater than 1:1 mole ratio of formaldehyde to phenol, respectively, a solids content of at least about 50 percent by weight, a pH between 5 and 9 and a viscosity at 77? F. between about 100 and about 1,000 centipoises and (11) between about 10 and 25 percent by weight, based on the weight of said resin, of an active powdered magnesium oxide having an initial setting time of less than about 6 hours.

4. The composition of claim 3 wherein the proportion of the fusible silica glass powder that is contained therein is in the neighborhood of 10 percent by weight, based on the weight of the composition.

5. The composition of claim 3 in the form of dry, free-flowing granules wherein the organic resinous binder material is in the form of solid particles intermixed-in the composition. a v

6. The composition of claim 3 in the form of a dry,

free-flowing powder wherein the resinous binder material 7. The composition of claim 3 in the form of a wet,

plastic and flowable mixture.

8. The composition of claim 3 wherein the sand has an AFS fineness number between about 75 and 180.

9. The composition of claim 3 in the form of dry, freeflowing granules.

10. The composition of claim 3 in the form of a wet, plastic and flowable mixture oxide.

11. A composition in accordance with the composition set forth in claim wherein the phenolic liquid resin is catalyzed with between about 10 and 25 percent by weight, based on the weight of the resin, of an active powdered magnesium oxide having an initial setting time between about 0.5 and 3 hours.

12. Method for the preparation of a wet composition that is particularly adapted to provide carbon-free shell molds and the like which consists essentially of dispersing a minor proportion of between about 3 and percent by weight, based on the weight of the composition, of a fusible silica glass powder in a binding quantity of an aqueous phenolic liquid resin and intimately incorporating the resulting dispersion in a mixture of a preponderant proportion of sand and a catalytic quantity of between about 10 and 25 percent by weight, based on the weight of resin in the composition, of an active powdered magnesium oxide having an initial setting time of less than about 6 hours for auto-hardening the phenolic liquid resin, said aqueous phenolic liquid resin consisting of a phenol-formaldehyde resin that has a greater than 1:1 mole ratio of formaldehyde to phenol, respectively, a solids content of at least about 50 percent by weight, a pH between 5 and 9 and a viscosity at 77 F. between about 100 and about 1,000 centipoises.

13. Method for fabricating carbon free shell molds and the like which consists essentially of forming a shell mold structure on a pattern from a composition that comprises an intimate mixture of a preponderant proportion of sand, a minor proportion of between about 3 and 15 percent by weight, based on the weight of the mixture, of fusible silica glass powder and a binding quantity of a thermoplastic-thermosetting organic resinous material; curing the formed shell mold structure of said composition at a thermosetting temperature to a rigid structure; and subsequently firing said thermoset pattern at the silica glass fusing temperature to fuse said silica glass powder for supporting said structure while burning out substantially all of the organic binder material from said structure, said resinous material consisting essentially of a mixture of (a) an aqueous liquid phenol-formaldehyde resin that has a greater than 1:1 mole ratio of formaldehyde to phenol, respectively, a solids content of at least about 50 percent by weight, a pH between 5 and 9 and a viscosity at 77 F. between about 100 and 10 about 1,000 centipoises and (b) between about 10 and 25 percent by weight, based on the weight of said resin, of an active powdered magnesium oxide having an initial setting time of less than about 6 hours.

14. The method of claim 13 wherein the thermoset shell mold pattern is fired at a silica glass powder fusing temperature between about 1740 and 1900 F.

15. The method of claim 14 wherein the composition is in the form of dry, free-flowing granules which is formed into said shell mold structure under the influence of a therrno-plastifying heat while it is in contact with said pattern.

16. The method of claim 14 wherein the composition is in the form of a wet, plastic and flowable mixture which is cold formed into said shell mold structure while it is in contact with said pattern.

17. The method of claim 14 wherein the composition is in the form of a wet, plastic and flowable mixture, said mixture being cold formed under pressure into said shell mold structure while it is in contact with said pattern before said resinous binder material has auto-hardened.

18. Carbon free shell mold obtained as the product of the process of claim 13.

19. Casting process for high temperature and low carbon metal alloys which consists essentially of casting the alloy under an inert atmosphere in a carbon free shell mold comprising a preponderant proportion of sand bound together in a rigid structure with a minor proportion of fused silica glass powder contained therein as a binder, said shell mold being obtained as the product of the process of claim 13.

References Cited in the file of this patent UNITED STATES PATENTS 2,683,296 Drumm et al. July 13, 1954 2,686,946 Jackson Aug. 24, 1954 2,688,169 Gruber et al. Sept. 7, 1954 2,692,246 Less et al. Oct. 19, 1954 2,696,479 Ossenbruggen et al. Dec. 7, 1954 2,705,822 Vennerholm Apr. 12, 1955 2,751,650 Froberger June 26, 1956 2,772,457 Webbere Dec. 4, 1956 2,797,457 Kramer July 2, 1957 FOREIGN PATENTS 305,237 Great Britain May 2, 1930 683,239 Great Britain Nov. 26, 1952 721,921 Great Britain Jan. 12, 1955 749,813 Great Britain May 30, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2 933 789 April 26 1960 Ronald H, Cooper It is herehfi certified that error appears in the-printed specification of the above "numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3 line l2 for "draying" read drawing column 3, line I? for "(AFC)" read (AFS) column 9 line 6 strike out "oxide". V

Signed and sealed this 29th day of November 1960,

( SEA L) Attest:

KARL H. AXLINE ROBERT c. WATSON Attesting Oflicer Commissioner of Patents 

1. COMPOSITION FOR THE FABRICATION OF CARBON-FREE SHELL MOLDS AND THE LIKE WHICH CONSISTS ESSENTIALLY OF A PREPONDERANT PROPORTION OF SAND, A MINOR PROPORTION OF BETWEEN ABOUT 3 AND 15 PERCENT BY WEIGHT, BASIED ON THE WEIGHT OF THE COMPOSITION, OF A FUSIBLE SILICA GLASS POWDER, AND A BINDING QUANTITY OF A THERMOPLASTICTHERMOSETTING ORGANIC RESINOUS MATERIAL, SAID RESINOUS MATERIAL CONSISTING ESSENTIALLY OF A MIXTURE OF (A) AN AQUEOUS LIQUID PHENOL-FORMALDEHYDE RESIN THAT HAS A GREATER THAN 1:1 MOLE RATIO OF FORMALDEHYDE TO PHENOL, RESPECTIVELY, A SOLIDS CONTENT OF AT LEAST ABOUT 50 PERCENT BY WEIGHT, A PH BETWEEN 5 AND 9 AND A VISCOSITY AT 77*F. BETWEEN ABOUT 100 AND ABOUT 1,000 CENTIPOISES AND (B) BETWEEN ABOUT 10 AND 25 PERCENT BY WEIGHT, BASED ON THE WEIGHT OF SAID RESIN, OF AN ACTIVE POWDERED MAGNESIUM OXIDE HAVING AN INITIAL SETTING TIME OF LESS THAN ABOUT 6 HOURS. 